The California Board of Prison Terms yesterday canceled a scheduled 1984 release date for Sirhan B. Sirhan, the assassin of Robert F. Kennedy, saying that Sirhan remains a menace to society because he has continued to issue death threats.

The three-member hearing board's decision came after 10 days of testimony and a statewide campaign in a crime-conscious election year to keep Sirhan behind bars.

The board said the decision of a 1975 board to release the Palestinian immigrant should be overruled because the earlier board had not been aware of threatening statements Sirhan had been making.

Announcing the decision in their Sacramento office under the glare of television lights, the board members pointed to a written threat to kill an author who wrote a book about him and a threatened murder if prison dentists did not fix his teeth.

Sirhan, who was informed of the decision yesterday at Soledad State Prison, must be given a new parole hearing within six months.

At that time the board must either give him a later parole date or rule that he is not yet fit for release.

"The prisoner has continued to verbalize threats," said board Chairman Ray Brown. "The panel finds that the continuing nature of the prisoner's threats and conduct has great significance. It is significant now and would have been to other panels had they had the information."

Earlier this year, according to testimony at Sirhan's parole hearing, he told a staff member at his medium-security prison, 120 miles south of San Francisco, "If my parole date is taken, I will turn the Arabs loose to get me out of prison, dead or alive."

If Sirhan were released on Sept. 1, 1984, as originally scheduled, his 16-year term would be three years longer than the average sentence for first-degree murderers in California sentenced to life terms, but it would be the minimum for life-term murderers under recently toughened state laws.

As Sirhan's expected parole date neared, politicians and citizens' groups organized letter and petition campaigns to stop his release, resulting in 10,000 letters, telegrams and petitions against him and a smaller but significant number in his favor.

Sirhan's attorney, Luke McKissack, has called the anti-Sirhan campaign a politically motivated act, since the official request to the board to revoke the parole date came from Los Angeles County District Attorney John Van de Kamp, a candidate for state attorney general.

Van de Kamp's principal opponent in the Democratic primary, state Sen. Omer Rains, has played a major role in getting the board to cancel paroles for two other notorious killers.

At the end of the lengthy board hearings last week, Sirhan made a dramatic, televised appeal to the board in which he said that if the presidential candidate he shot down in hotel kitchen corridor in 1968 were alive today, "He would not countenance singling me out for this kind of treatment. I think he would be among the first to say that no matter how horrible a deed I committed 14 years ago, that should not become the cause for denying me equal treatment under the laws of the country."

McKissack said yesterday that Sirhan had no reaction to the decision, other than to ask a technical question that McKissack promised to research.

Sirhan, who indicated that he killed Kennedy on the night of his victory in the California presidential primary because of the senator's pro-Israel stance, has said he hopes to move to an Arab country when he is released, perhaps Jordan, where his father lives.

In 1969 a Los Angeles jury sentenced Sirhan to die in the gas chamber, but in 1972 the state supreme court ruled the death penalty unconstitutional and Sirhan was given a life term with possibility of parole.

The parole board said yesterday, however, that the earlier board erred in thinking it had to set a release date immediately for Sirhan because of the court decision